·A pond is an example of a complete, closed and an independent
ecosystem.

·It is convenient to study its basic structure and functions.

·It works on solar energy and maintains its biotic community
in equilibrium.

·If you collect a glass full of pond water or a scoop full of pond bottom
mud, it consists of a mixture of plants, animals, inorganic and organic
materials.

vFollowing components are found in a pond ecosystem.

(a) Abiotic components :

(i) Light:

·Solar radiation provides energy that controls the
entire system.

·Penetration of light depends on transparency of water, amount of
dissolved or suspended particles in water and the number of plankton.

·On the basis of extent of penetration of light a
pond can be divided into euphotic
(eu=true,photic=light), mesophotic and aphotic zones.

·Plenty of light is available to plants and animals in euphotic
zone.

·No light is
available in the aphotic zone.

(ii) Inorganic substances:

·These are water, carbon, nitrogen, phosphorus, calcium and a few
other elements like sulphur depending on the location of the pond.

·The inorganic substances like O2 and
CO2 are in dissolved state in water.

·All plants and animals depend on water for their food and exchange
of gases- nitrogen, phosphorus, sulphur and other inorganic salts are held in
reserve in bottom sediment and inside the living organisms.

· A very
small fraction may be in the dissolved state.

(iii) Organic compounds:

·The commonly found organic matter in the pond are amino acids and humic acids and the breakdown products of
dead animals and plants.

·They are partly dissolved in water and
partly suspended in water.

(b) Biotic components :

(i) Producers or autotrophs:

·synthesize food for all the heterotrophs of the pond.

·They can be categorized into two groups:-

(a) Floating
microorganisms and plants

(b) Rooted plants

(a) Floating microorganisms (green) and plants are called phytoplankton

(“phyto”- plants, “plankton” –floating).

·They are microscopic organisms. Sometimes they are so abundant in
pond that they make it look green in colour e.g. Spirogyra, Ulothrix, Cladophora, Diatoms, Volvox.

(b) Rooted plants:

·These are arranged in concentric zones from periphery to the
deeper layers.

·Three distinct zones of aquatic plants can be seen with increasing
depth of water in the following order:

·At every step in a food chain or web the energy received by the
organism is used to sustain itself and the left over is passed on to the next
trophic level.

Ecological pyramid and its types :

·Ecological pyramids are the graphic representations of trophic levels in an
ecosystem.

(1) Pyramid of number:

·This represents the number of organisms at each trophic level.

·For example in a grassland the number of grasses is more than the
number of herbivores that feed on them and the number of herbivores is more
than the number of carnivores.

·In some instances the pyramid of number may be inverted, i.e
herbivores are more than primary producers as you may observe that many
caterpillars and insects feed on a single tree.

(2) Pyramid of biomass:

·This represents the total standing crop biomass at each trophic level.

·Standing crop biomass is the
amount of the living matter at any given time.

·It is expressed as gm/unit area or kilo cal/unit area.

·In most of the terrestrial ecosystems the pyramid of biomass is
upright.

·However, in case of aquatic ecosystems the pyramid of biomass may
be inverted e.g. in a pond phytoplankton are the main producers, they have very
short life cycles and a rapid turn over rate (i.e. they are rapidly replaced by
new plants).

·Therefore, their total biomass at any given time is less than the
biomass of herbivores supported by them.

(3) Pyramid of energy:

·This pyramid represents the total amount of energy at each trophic
level.

·Energy is expressed in terms of rate such as kcal/unit area /unit
time or cal/unit area/unit time.eg.

·in a lake autotroph energy is 20810 kcal/m/year

·Energy pyramids are never inverted.

ECOLOGICAL EFFICIENCY :

·It is clear from the trophic structure of an ecosystem that the
amount of energy decreases at each subsequent trophic level. This is due to two
reasons:

1. At each trophic a
part of the available energy is lost in respiration or used up in

metabolism.

2. A part of energy
is lost at each transformation, i.e. when it moves from lower to higher trophic
level as heat.

·It is the ratio between the amount of energy acquired
from the lower trophic level and the amount of energy transferred from higher trophic
level is called ecological efficiency.

·Lindman in 1942
defined these ecological efficiencies for the 1st time and proposed 10% rule e.g. if autotrophs produce 100 cal,
herbivores will be able to store 10 cal. and carnivores 1cal. However, there
may be slight variations in different ecosystems.

·In ecosystems flow of energy is linear but that of
nutrients is cyclical.

·This is because energy flows down hill i.e. it is
utilized or lost as heat as it flows forward The nutrients on the other hand
cycle from dead remains of organisms released back into the soil by detrivores which
are absorbed again.

· i.e. nutrient absorbed from
soil by the root of green plants are passed on to herbivores and then
carnivores. The nutrients locked in the dead remains of organisms and released
back into the soil by detrivores and decomposers.

·There are more than 40 elements required for the various life
processes by plants and animals.

·The entire earth or biosphere is a closed system
i.e. nutrients are neither imported nor exported from the biosphere.

·There are two important components of a biogeochemical cycle

(1) Reservoir pool- atmosphere or rock, which stores large amounts of nutrients.

(2) Cycling pool or compartments of cycle-They are relatively short storages of carbon in the form of
plants and animals.

Carbon cycle :

·The source of all carbon is carbon dioxide present in the
atmosphere.

·It is highly soluble in water; therefore, oceans
also contain large quantities of dissolved carbon dioxide.

The global carbon cycle consists of following steps-

Photosynthesis
:

·Green plants in the presence of sunlight utilize CO2 in the
process of photosynthesis and convert the inorganic carbon into organic matter
(food) and release oxygen.

·A part of the food made through photosynthesis is used by plants
for their own metabolism and the rest is stored as their biomass which is
available to various herbivores, heterotrophs, including human beings and
microorganisms as food.

·Annually 4-9 x1013 kg of CO2 is fixed by green plants
of the entire biosphere.

·Forests acts as reservoirs of CO2 as carbon fixed by the trees
remain stored in them for long due to their long life cycles.

·A very large amount of CO2 is released through
forest fires.

Respiration :

·Respiration is carried out by all
living organisms.

·It is a metabolic process where food is oxidized to
liberate energy, CO2 and water.

·The energy released from respiration is used for carrying out life
processes by living organism (plants, animals, decomposers etc.).

·Thus CO2 is released into of the atmosphere through
this process.

Decomposition
:

·All the food assimilated by animals or synthesized by plant is not
metabolized by them completely.

·A major part is retained by them as their own biomass which
becomes available to decomposers on their death.

·The dead organic matter is decomposed by microorganisms and CO2 is
released into the atmosphere by decomposers.

Combustion
:

·Burning of biomass releases carbon dioxide into the atmosphere.

Impact of human activities :

·The global carbon cycle has been increasingly disturbed by human
activities particularly since the beginning of industrial era.

·Large scale deforestation and ever growing consumption of fossil
fuels by growing numbers of industries, power plants and automobiles are
primarily responsible for increasing emission of carbon dioxide.

·Carbon dioxide has been continuously increasing in the atmosphere
due to human activities such as industrialization, urbanization and increasing
use and number of automobiles.

·This is leading to increase concentration of CO2 in the
atmosphere, which is a major cause of global warming.

Nitrogen cycle :

·Nitrogen is an essential component of protein and
required by all living organisms including human beings.

·Our atmosphere contains nearly 79% of nitrogen but
it can not be used directly by the majority of living organisms.

·Broadly like corbondioxide, nitrogen also cycles from gaseous phase
to solid phase then back to gaseous phase through the activity of a wide
variety of organisms.

·Cycling of nitrogen is vitally important for all living organisms.

·There are five main processes which essential for nitrogen cycle
are elaborated below.

(a) Nitrogen fixation:

·This process involves conversion of gaseous nitrogen into Ammonia, a form in which it can be
used by plants.

·Atmospheric nitrogen can be fixed by the following three methods:-

(i) Atmospheric fixation:

·Lightening, combustion and volcanic activity help in the fixation of nitrogen.

(ii) Industrial fixation:

·At high temperature (400oC) and high pressure (200
atm.), molecular nitrogen is broken into atomic nitrogen which then
combines with hydrogen to form ammonia.

·Precipitation (rain, snow, slush dew etc.) is the only source of
water on the earth. Water received from the atmosphere on the earth returns
back to the atmosphere as water vapour resulting from direct evaporation and
through evapotranspiration the continuous movement of water in the biosphere is
called water cycle (hydrological cycle).

·Water is not evenly distributed throughout the surface of the
earth.

·Almost 95 % of the total water on the earth is chemically bound to
rocks and does not cycle.

·Out of the remaining 5%, nearly 97.3% is in the oceans and 2.1%
exists as polar ice caps.

·Thus only 0.6% is present as fresh water in the form of
atmospheric water vapours, ground and soil water.

·The driving forces for water cycle are

1) solar radiation

2) gravity .

·Evaporation and precipitation are two main processes involved in
water cycle.

·These two processes alternate with each other Water from oceans,
lakes, ponds, rivers and streams evaporates by sun’s heat energy.

·Plants also transpire huge amounts of water.

·Water remains in the vapour state in air and forms clouds which
drift with wind.

·Clouds meet with the cold air in the mountainous regions above the
forests and condense to form rain precipitate which comes down due to gravity.

·On an average 84% of the water is lost from the surface of the
through oceans by evaporation.

·While 77% is gained by it from precipitation. Water run-off from lands through rivers to oceans makes up
7% which balances the evaporation deficit of the ocean.

·On land, evaporation is 16% and precipitation is
23%.

HOMEOSTASIS OF ECOSYSTEM :

·Ecosystems are capable of maintaining their state of
equilibrium.

·They can regulate their own species structure and functional
processes.

·This capacity of ecosystem of self
regulation is known as homeostasis.

·In ecology the term applies to the tendency for a biological systems
to resist changes.

·For example, in a pond ecosystem if the population of zooplankton
increased, they would consume large number of the phytoplankton and as a result
soon zooplankton would be short supply of food for them.

·As the number zooplankton is reduced because of starvation,
phytoplankton population start increasing.

·After some time the population size of zooplankton also increases
and this process continues at all the trophic
levels of the food chain.

·Note that in a homeostatic system, negative feedback mechanism is
responsible for maintaining stability in a ecosystem.

·However, homeostatic capacity of ecosystems is not
unlimited as well as not everything in an ecosystem is always well regulated.